While diseases such as cancer are due to mistakes occurring in genes during our lifetimes, there are many diseases - more than 4,000, with more being discovered all the time - that are caused by a fault in just one gene, and that are present from birth. The majority of these "single gene disorders" are very rare diseases. Some may only affect a few tens of people worldwide. But others, including cystic fibrosis, are relatively common.

Because we inherit our genes in pairs, with one copy from each of our parents, provided one of the pair is OK, we will still be healthy, even if the other copy is a dud. All of us carry dud genes, but we'll never know this - unless, that is, we are unfortunate enough to have a baby with someone who also has that "one good, one bad" combo in the very same gene that we do.

Every baby born of this union has a 1 in 4 chance of inheriting two bad copies. And if both copies are bad, the baby's health may be affected by genetic disease. But just one good copy prevents this. It helps explain why some parents who have a baby affected by a single gene disorder will have no family history of the disorder. It also explains why, even though 1 in 25 British people are estimated to be carriers of the cystic fibrosis gene, the number of people who are actually affected by the disease is only 1 in 2,500.

Diseases caused by single gene disorders are passed from one generation to another in a very predictable way, enabling geneticists to draw up detailed family pedigrees.

The pattern of inheritance seen in cystic fibrosis is called autosomal recessive - autosomal means that a gene is not located on either of the X or Y sex chromosomes but on one of the other 22 pairs. Another pattern is called autosomal dominance. A classic example is Huntington's disease. Here the instruction from the bad gene prevails, even if a good copy is present. And there is a 50:50 chance that it will be passed to each child.

Sex-linked disorders

Sex-linked genetic diseases are those where the defective genes are carried on either the X or Y chromosomes. They mostly affect boys. This is because boys only have one X chromosome, so a single recessive gene on that X chromosome will cause the disease. A classic example of this is seen in diseases like haemophilia. Although women can get X-linked recessive disorders, it is very rare and they are usually only carriers.

But how about genes that go wrong in your lifetime? Every time your DNA - the fabric of your genes - is copied, there's a chance that a small error can be made in the copying. Even a small change in the sequence of base pairs can cause havoc. Imagine mishearing an instruction that said: "bring a hat" instead of "bring a cat". These alterations are called mutations, and can accumulate over a lifetime. Errors in genes that control cell division can cause cancers.

For a cell to become cancerous, a number of genetic mutations have to take place. Some people are born with one of these mutations, such as BRCA1 for breast cancer. This is called a genetic predisposition; but even so, it is not certain carriers of these genes will get breast cancer. Each cell contains powerful repair mechanisms for damaged DNA. However, these are less efficient as we get older, which is why cancer is more likely as we age.

There are also errors arising from damage to DNA, which is under constant assault. Inside the cell it is damaged by the harmful byproducts of life-sustaining chemical reactions. Outside the cell, many things, including sunlight, tobacco smoke and radiation, can damage DNA.